Multiplexing is a technique that allows a plurality of channels to be transmitted via a common channel or path. Time-division multiplexing allows the plurality of channels to share the common channel by assigning different ones of the plurality of channels to use the common channel at different times. In statistical multiplexing, the plurality of channels are connected to the common channel on a statistical basis, i.e., according to need. Typically, the signals from different ones of the plurality of channels that are successively transmitted on the common channel are separated on the common channel by idle periods of arbitrary duration. Consequently, the signals must be delimited by identifiers that indicate when the signals start and stop. Also, there is no a priori, predetermined, sequence in which the channels are connected to the common channel. Consequently, the signals from any one of the plurality of channels that are transmitted on the common channel during any time interval must include explicit addressing information that identifies either the source or the destination of those signals.
In contrast, in subrate multiplexing, the plurality of channels are connected to the common channel during adjacent successive time intervals, or slots, in a predetermined order that forms a repeating pattern of fixed duration, called a frame. Because the time slots are adjacent, i.e., not separated by idle intervals, transmissions of signals from individual ones of the plurality of channels are delimited by the beginning and end of the individual time slots and hence need no other delimiters. Also, any changes in the relative order of transmission on the common channel, such as substitution of one or more of the plurality of channels for another one or more of the plurality of channels in using one or more of the time slots of a frame, must be made known by the transmitter (the multiplexer) and to the receiver (the demultiplexer) prior to the change being effected. Consequently, the one of the plurality of channels that is the source of the signals transmitted during any time slot is identifiable from the position of those signals within the frame, and hence the signals need not include explicit addressing information.
Subrate multiplexing is well known in the art. Typically, it involves the combining through time-division techniques of a plurality of independent communication signal streams intended for transmission over a common communication channel and each one of which has a bandwidth, i.e., a transmission rate, that is lower than, i.e., that is a subrate of, the bandwidth of the common channel, into a single signal stream having the common channel's bandwidth. For example, it is common to combine the signal streams of up to 20 subchannels, each having a bandwidth of 2.4 kilobits per second (Kbps) plus associated control signals (e.g., framing information), into a single ISDN B or DS0 channel signal stream having a total bandwidth of 64 Kbps or 56 Kbps, respectively, by using a different one of each 20 successive time slots of the ISDN B or DS0 channel to carry the data signals and associated control signals of a different one of the 20 2.4 Kbps subchannels.
Subrate multiplexing is effected via devices appropriately referred to as subrate multiplexers. Subrate multiplexers have hitherto performed the multiplexing functions in hardware, i.e., via special-purpose circuits designed specifically for performing the multiplexing functions. This has made the subrate multiplexers inflexible, in that they have not been capable of adapting to different communication protocols, rates, and services or functions, without a change in their hardware, i.e., in their circuitry.
To overcome some of this inflexibility, microprocessors have been incorporated into some subrate multiplexers for the purpose of controlling the operation of the multiplexer hardware. But even then, the subrate multiplexers have continued to perform the subrate multiplexing functions via the special-purpose circuits, i.e., via hardware. The wisdom of the art is that subrate multiplexing must be done in hardware.